She's still smiling after 500 years, but the strain is starting to show. Mona Lisa, perhaps the most famous painting of all, is showing signs of succumbing to the ravages of time.

Curators at the Louvre warned yesterday that Leonardo da Vinci's masterpiece was deteriorating quickly, and they have commissioned a detailed study to work out why.

The smile remains as enigmatic as ever, but the thin panel of poplar wood on which it is painted is "more warped than it was previously", the museum said, adding that the development was the cause of "some worry".

A technical study of the painting will assess its vulnerability and determine what materials it is made of. The artwork already undergoes evaluation every one or two years.

"These analyses will take place in such a way as to allow the work to remain on public display," the Louvre said. The tests are due to begin in 2005, when the painting will move to a gallery of its own following a £2.5m refurbishment.

The most analysed portrait in the history of art, the Mona Lisa is seen by nearly all of the 6 million people who visit the Louvre each year, the museum says. It is believed to have been painted over a long period, beginning in about 1505. The work set the standard for portrait painting for centuries to come.

The admiration it has always evoked is attributed to a number of factors: fascination with Da Vinci's brilliance, genius and persona; the artwork's stunning realism and technique; the mystery of the subject's true identity; and the twists and turns in its history.

In 1911 an Italian painter stole the picture from the Louvre to get it back into Italian hands. After a lengthy police inquiry that involved a long list of suspects including the French poet Guillaume Apollinaire, the painting was found in Italy two years after it disappeared.

The theft was splashed across newspaper front pages around the world - something that helped to raise the picture's fame to a higher level.

The Mona Lisa has taken on a brownish cast due to the accumulation of dust and dirt and chemical changes to the varnish covering its surface, but the museum has so far resisted pressure to try to restore its original colours.